1. Field of the Invention
The present invention relates to a television signal scanning conversion device. Specifically the present invention concerns a device comprising at least one filtering block having a plurality of digital inputs which receive, through an interface, components of a three-dimensional television signal.
2. Discussion of the Related Art
In this specific technical area it is known that image quality improvement in television receivers is correlated with the amplitude of the signal band. A problem which limits the good quality of the video signals is the reduced amplitude of the signal band.
Already upon generation of the television signal, e.g., inside the television camera, there arise problems related to the occupation of the signal band.
It is important that the signal reproduce the scene in the best possible manner; which signifies a compromise solution between reproduced image quality and small band occupation. Various compromise solutions of this type have then been translated into different standards which however still use a particular combination of time and space sampling which leads to the constitution of an image having a so-called interlaced format.
While fulfilling the purpose, this sampling combination introduces also some artifices well known to those skilled in the art and known as `interline flicker`, `large area flicker`, `line crawl` and `raster visibility`.
To obviate these shortcomings it is possible to use some measures based essentially on interpolation algorithms which take from the receiver the missing information without increasing the transmission band.
Before discussing the aspects linked to the use of these interpolation processes, it should be recalled that the television signal is a three-dimensional signal having two spatial dimensions, i.e. one horizontal and the other vertical. A third time dimension allows reconstruction of the motion of the scene shot and reproduction then of images in motion. We shall define below as an image field an assembly of rows which constitute a television image in a same time interval equal to the field scanning time. The number of rows in a field is equal to half the total number of rows and therefore an `odd field` and an `even field` are distinguished depending on whether each field consists of the odd or even rows of the image. The term `frame` indicates the whole of two fields spaced by a time interval equal to the time sampling period.
The time extension of course permits reproduction of the motion which characterizes the objects present in the scene shot. But because of the motion it is not possible to utilize the highest resolution of the raster by superimposing two fields. Between the fields, indeed, there is a time interval equal to a time sampling period and hence they are in general for two different images, at least if an object has moved.
Therefore the superimposition of the two fields, if there has been motion, is not an acceptable solution just because different images are superimposed.
Returning now to the problem of limited band amplitude, this problem must be considered with reference to the three dimensions of the signal and the use of the interpolation processes which allow operation of a space-time filtering of the signal. These interpolation processes fall into two broad classes depending on how they allow for the image movement information,
A first class consists of the processes which do not allow for movement and hence make use of the information present in the same field and therefore are called `intrafield` type.
The second class consists of processes which make use of the information present in two consecutive fields, detecting the presence of motion, and are called `interfield` type.
If in the image sequence there is no movement, the highest possible resolution is obtained by superimposing the two fields (field insertion). But if there is movement, the highest possible resolution is obtained by taking the missing information from the pixels belonging to the same field (intrafield) allowing for the presence of special structures such as for example fronts.
These problems linked to television signal interlacing can be considered as related to the attempt to improve the resolution of the signal. The known art already proposes some solutions which are basically intended to increase signal resolution.
For example, the time resolution can be increased by doubling the image field frequency by means of a scanning rate converter (SRC). There are in particular two classes of processes of this kind:
1. Interlaced-to-progressive conversion (IPC) which aims at increasing the spatial resolution by doubling the number of rows for each field, and
2. frame-rate-up conversion (FRU) which aims at increasing the time resolution by increasing the number of fields shown in the time unit.
In any case, whether application of an IPC or an FRU procedure is desired, a shortcoming is that it is necessary to find the spatial information, i.e. the values of the missing pixels. Therefore, while advantageous in some ways, these techniques do not yet provide good filtering by adaptive procedures.